Riblet Foil and Method for Producing Same

ABSTRACT

A method for producing a layer-type buildup of riblet foil involves applying a metal powder is to a reference mold, which has a female mold of a riblet structure, in such a manner that a metallic material is formed. The layered portion applied in this manner is then detached, thereby forming a riblet foil having a riblet structure.

FIELD OF THE INVENTION

Exemplary embodiments of the present invention relate to a riblet foil and to a method for producing same. In particular, the riblet foil is made of metallic material and composed by way of layering said material onto a reference mold. The riblet foil has a riblet structure that reduces air resistance.

BACKGROUND OF THE INVENTION

Lowering fuel consumption and reducing carbon dioxide/nitrogen oxide (CO₂/NOx) emissions are important objectives to be met, particularly by the aviation and automotive industries. Aside from light construction modalities to achieve weight reductions and improvements to the efficiency of the drive systems, reducing air resistance constitutes one essential component in achieving these goals.

Methods for producing surfaces that reduce the resistance to air flow are known in the art. It has been known for a long time, and, moreover, it has been proven in oil channel experiments, that, by providing solid surfaces with suitable structures, it is possible to reduce the drag of liquid or gaseous media flowing over such solid surfaces. The structure or texture of the solid surface therein can come in different forms and sizes. Bechert, D. W. et al, Experiments on drag-reducing surfaces and their optimization with adjustable geometry, Journal of Fluid Mechanics, Vol. 338, pp. 59-97, 1997 teach gluing a structured plastic foil onto bodies.

Relying on this information, aviation companies, such as Airbus and Boeing, conducted experiments with plastic foils made by 3M that had corresponding structures disposed thereupon, and which were then glued to the exterior surfaces of an aircraft. In flight experiments with a large aircraft that had 70% of the surface thereof covered with such glued-on plastic foil, it was possible to document a 2% drag reduction (Szodruch J., Dziomba, B., Aircraft Drag Reduction Technologies, 29th Aerospace Science Meeting, Reno, Nevada, Jan. 7-10, 1991). This resulted in fuel savings of 1.5% (Roberts J. P., Drag Reduction: An Industrial Challenge, Special Course on Skin Friction Drag Reduction, AGARD Report 786, 1992).

However, structured plastic foils that have been used to date lack mechanical durability and suffer from degradation of the plastic foil as well as of the glue by which these foils are to adhere to the aircraft. One consequence of the lack of mechanical durability is, for example, a rounding of the tip radii in saw-tooth-shaped structures, which can completely neutralize the friction-lowering effect (Hage, W., Zur Widerstandsverminderung von dreidimensionalen Ribletstrukturen und anderer Oberflächen [Regarding Drag Reduction of Three-dimensional Riblet Structures and other Surfaces], Dissertation Technical University of Berlin 2004). Degradation effects can even cause the plastic foils to become detached while in operation.

An as of yet unpublished patent application by the applicant offers a description specifying that, with the use of structured or textured metal foils, for example on a structural component of an aircraft or helicopter, it is possible to achieve a considerable reduction in the resistance to air flow. To this end, the structure of the metal foil includes riblets, which are microscopically small grooves in the surface of the metal foil. Such textured metal foils will be referred to below as “riblet foils.” The mechanical resistance of riblet foils is greater than that of plastic foils. Using riblet foils on fiber-reinforced plastic structures simultaneously provides electric conductivity for lightning protection.

German patent document DE 10314373A1 discloses a precision-casting method for the production of the structured metal foils that is involved in the production of turbine vanes made of titanium aluminums (TiAl) having textured surfaces. Applying the structure to the metallic surfaces is also possible by laser processing or a grinding process, known in the art based on the description in, for example, in Oehlert et. al., Exploratory Experiments on Machined Riblets for 2-D Compressor Blades, Proceeding[s] of IMECE2007, 2007 ASME Internation[al] Mechanical Engineering Congress and Exposition, Nov. 11-15, 2007, Seattle, Wash., United States.

However, such a direct application of a structure or texture by means of casting method or by removal-machining methods (structure generation by a laser or micro-milling means, etc.) does not make technical or economic sense for use on the large areas, as are required on aircraft, helicopters or other flying devices.

On the other hand, manufacturing methods for producing the required quantity and/or size of riblet foils are known in the art, where metal foils are structured/textured by mechanical processing. For example, these are mechanical (re)shaping methods, such as rolling (including incremental methods, such as incremental rolling) or stamping.

To create sharp-edged structures on the metallic foil, it is necessary for the used tools, such as, for example, the rollers, to pressed against the foil with a high pressure. Tools such as this are usually formed of a solid full body, such as, for example, a steel body. Moreover, such tools must be manufactured of a corresponding material, if it is necessary, for example, to work at elevated temperatures to modify the metallic foil by means of a plastic deformation, and thereby creating a riblet foil. High-strength materials, such as, for example, titanium alloys with tensile strength values of up to 1700 MPa, can no longer be sufficiently plasticized by the application of a deformation pressure, whereby the riblet structure cannot be achieved at all, or only coarsely, on the metal foil.

Therefore, exemplary embodiments of the present invention provide a riblet foil and a method for manufacturing the same that overcomes the disadvantages of the prior art, that is cost-effective and involves minimal technical complexity in terms of use.

SUMMARY OF THE INVENTION

Exemplary embodiments of the present invention are directed to providing a simple method for producing metallic riblet foils, wherein the mechanical and functional properties of the riblet foil can be adapted to the requirements of the respective application by a suitable selection from among the most varied metals and/or metallic alloys. The riblet foil is produced by the buildup of a layered structure onto a reference mold and then detached from the same. The reference mold includes at least on one surface a structure that is a female mold of the at least one riblet texture. The female mold includes indentations of microscopic size in the surface of the reference mold. After detaching the reference mold, riblets remain behind as microscopically small elevations/tips on the formed riblet foil. The riblets can be formed on the totality of the riblet foil or in selected partial areas. The surface of the reference mold is textured correspondingly.

In other words, the present invention provides, independently of the required size, form or properties of the riblet foil, a simple method that allows for producing a riblet foil having a desired form, size as well as any desired properties from any metallic powder. The thickness of the riblet foil itself, as well as the size and shape of the riblets, are variably producible using the method according to the invention. Advantageously, the thickness of the riblet foil is less than 500 μm; especially advantageously, the thickness is between 50-400 μm, particularly at least 200 μm. The thickness of the riblet foil ensures that the foil easily adjusts itself to the respective structural component to which it is applied, while having a minimally possible weight.

The invention involves a method for producing a riblet foil, wherein the riblet foil is composed in layers. A reference mold has a structure/texture at least on one surface, which corresponds to a female mold of at least one riblet structure. At least one layered section comprised of a metallic powder is applied in such a manner to this area that a metallic material is formed. The application of further layered portions of metallic powder in a vertical or horizontal direction onto the surface of the reference mold can be repeated as often as desired. The at least one layered portion thus obtained is detached from the reference mold, and whereby the metallic riblet foil is formed. The same has a riblet structure at least on one surface.

The metal or metal alloy powder comprises at least all light metals in the main groups and sub-groups of the periodic system; in particular, aluminum or titanium and mixtures thereof, wherein it is also possible for reinforcing particles to be mixed in with the metal/metal alloy powder, as well as hard metals. The hard metals comprise at least one compound material that is part of a reinforcement phase, such as tungsten carbide (WC), and a matrix or binder, such as cobalt. It is understood that the metals also comprise metallic alloys that contain at least one of the mentioned metals, including copper, zinc, lithium, beryllium, scandium, vanadium or yttrium, and, in particular, also steel alloys; preferred are titanium/titanium alloys or steel alloys, in particular aluminum/aluminum alloys.

The reference mold can be made of a wide variety of materials. Preferred is a reference mold, or surface that has the mentioned structure/texture, which is formed of materials having a high durability and that demonstrate very low wear and tear, such as, for example, steel X45NiCrMo4, that is known from use in stamping and shaping tools and has good polishing properties.

The area of the reference form comprises a structure corresponding to a female mold of at least one riblet structure, which is formed by a plurality of trapezoid, semi-circular, streamlined and/or saw-tooth-shaped riblets or a combination thereof. The female molds of the riblets comprise a size range with a riblet width of approximately 20 μm to 130 μm, preferably 30 μm to 80 μm, particularly 50 μm to 60 μm, especially 60 μm. At least one female mold of a riblet has a height that is half the distance to the next adjacent riblet thereto.

The application of the layered portion to the area of the reference mold is achieved in layers. It is possible to apply one or a plurality of layers in succession of the same metal powder or of different metal powders. Advantageously, the first layered portion that is applied can be a layer of titanium powder (or titanium alloy powder), and the remaining layered portions are of aluminum powder. The thus formed riblet foil has very robust, durable riblets and a base that can be produced cheaply from aluminum. Moreover, the person skilled in the art can provide the aluminum, using methods that are known in the art, such as anodizing, with a surface that is ideally suited for the gluing action.

The properties of the textured surface, in particular good polishing characteristics, facilitate the easy detachment of the one or more applied layered portion(s) from the reference mold.

The metallic powder is preferably applied to the reference mold by a spray-on method. For example, the metallic powder is applied in layers by a cold gas spray process, a thermo-kinetic application process (for example, Flamecon® by Leoni), a plasma-supported application process (for example, Plasmadust® by Reinhausen Plasma) or known thermal spray application processes, such as a powder process, a plasma spray process or a High Velocity Oxygen Fuel (HVOF) process. These processes allow for creation of textured structures having μm-geometries. The formed riblet structures comprise a size range with riblet widths of approximately 20 μm to 130 μm, preferably 30 μm to 80 μm, particularly 50 μm to 60 μm. The height of the riblet comprises a range of 50-100% of the riblet width. The riblet height can also be equal to the riblet width, or it can comprise a combination from the indicated range of sizes. The structure of the female mold is formed correspondingly on the reference mold.

When building up at least one layered portion by a spray-on process, the metal powder is accelerated, with high energy, toward the surface of the reference mold. Upon colliding with the surface, the metallic particles become connected to one another, due to a plastic deformation or cold welding process, thus forming a metallic material. During the cold-welding process, a heated gas is accelerated for this purpose, and the metallic powder is injected into the gas jet, thereby accelerated. During plasma spray processes, as an example for thermal spray methods, a light arc is generated between the cathode and anode of the plasma burner, either under a normal or inert atmosphere or in a vacuum. The gas flowing through the plasma burner is directed through the light arc and ionized. The metallic powder is jet-sprayed into this heated gas, then molten/plasticized due to the high plasma temperature. The plasma flow accelerates the metallic particles in the direction of the area of the reference mold. After colliding with the reference mold, the metallic particles combine into a dense, solidly adhesive layer due to plastic deformation and/or cold welding and/or solidifying.

Spraying on the at least one layered portion has the advantage that a much higher precipitation rate can be achieved on the reference mold than, as compared to, for example, galvanic deposition processes according to the prior art. Aside from the economic profitability in terms of production, this process also allows, in particular, for the generation of large riblet foils that can be applied to the wings of aircraft, such as the wings of an A380 model that is built by Airbus Operations GmbH. Moreover, it is possible to select almost any conceivable metallic powder, wherein the grain sizes can vary and the size of the riblet structure can be selected correspondingly ranging from only very few nm to several hundred μm.

The at least one layered portion can have a differing thicknesses. The thickness of the layer can be variable; for example, the edge regions can be thicker or thinner than the center region of the layered portion. It is also possible to form the totality of the layered portion as having a substantially constant thickness.

The reference mold can be disposable or envisioned for several uses. A reusable reference mold is cost-effective and can be employed in the production of riblet foils of different metallic powders, wherein the riblet structures are identical.

In a further embodiment, the reference mold is configured as a roller. This facilitates a continuous production of the riblet foil according to the invention.

Advantageously, the thickness of the riblet foil is adjustable by means of the rotational speed of the roller. When the rotational speed is low, thicknesses of, for example, 400 μm are achieved. When, on the other hand, the riblet foil is to be thin, for example 200 μm, the rotational speed of the roller is simply increased. This way, regulating the thickness of the riblet foil is facilitated by simple means.

The metallic riblet foil can be refined, at least in part. This process comprises, for example, an adhesive coating that is applied to the back side, functionalization of the surfaces by way of a coating or modification of the surface, thereby providing the same with fastness properties against contamination or icing.

BRIEF DESCRIPTION OF THE FIGURES

The invention will be described below in an exemplary manner based on the embodiments that are depicted in the figures. For better clarity, the embodiments are not drawn to scale.

FIG. 1 a shows the production of a riblet foil according to a first embodiment;

FIG. 1 b shows the detached riblet foil with a trapezoid riblet structure;

FIG. 2 depicts the production of the riblet foil according to a second embodiment.

EMBODIMENTS OF THE INVENTION

FIG. 1 a is a sketch-type depiction of a riblet foil. A reference mold 11 includes an area 12 and a riblet structure 13. Using a coater system 16, metallic particles 15 of a metallic powder are accelerated to a high speed in the direction toward the area 12. Due to the impact, the metallic particles 15 become deformed and combine to form a metallic material. A continuous spraying action of metallic particles 15 against the area 12 results in a layer-type buildup of a layered portion 14. When the thickness of the layered portion 14 has reached the thickness that is required for the application the spray-on action of metal powder is complete.

In this embodiment, the coater system 16 is a system for cold gas spraying. A carrier gas is compressed and accelerated by relaxation of the same inside a nozzle to a speed that is below the speed of sound of the carrier gas. The metallic powder is injected into the gas jet. The carrier gas has a temperature that is below the temperature at which the metallic particles 15 melt, such that these particles do not, in fact, melt. The metallic particles 15 become deformed upon colliding with the area 14, wherein the thus resulting localized heat release ensures the cohesion and adhesion needed, and thus forming the layered portion 14, whereby a metallic material is formed.

FIG. 1 b shows the now detached riblet foil having a trapezoid riblet structure.

The riblet foil 10 has a base 18 and a riblet structure 17 with trapezoid riblets. The reference mold 11 is reusable in the production of a further metallic riblet foil 10.

FIG. 2 shows the production of a riblet foil using a roller 11 as reference mold. On the area 12, the roller 11 includes a female mold of a riblet structure 13, which has been omitted in the drawing for better clarity.

Using the coater system 16, metallic particles 15 are accelerated to high speeds, as described previously in connection with FIG. 1, and collide in a high kinetic energy state with the area 12 of the roller 11. Due to the plastic deformation of the metallic particles 15 and cold welding, the metallic particles 15 are combined into a dense layered portion 14, thereby forming a metallic material.

The rotational speed of the roller 11 determines the thickness of the layered portion 14. At a low rotational speed of the roller 11, the metallic particles 15 encounter a previously formed layer, thereby creating an overall greater thickness of the layered portion 14. The thickness of the layered portion 14 depends on several factors, such as, for example, the circumference of the roller 11 and the speed of the coater system relative to the roller 11.

When the rotation of the roller 11 is continuous, and the metallic powder is sprayed on continuously by means of the coater system 16, a continuous layered portion 14 is produced. This portion is taken up by the unwinding roller 19 and detached from the roller 11 by the rotation of unwinding roller 19 against the direction of rotation of the roller 11.

The continuous riblet foil 10 has a base 18 and a riblet structure 17, which have been omitted from FIG. 2 for better clarity and which are comparable to the riblet structure 17 as illustrated in FIG. 1 b.

The foregoing disclosure has been set forth merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to persons skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.

LIST OF REFERENCE SIGNS

-   10 Riblet foil -   11 Reference mold, roller -   12 Area -   13 Riblet structure -   14 Layered portion -   15 Metallic particle -   16 Coater system -   17 Riblet structure -   18 Base -   19 Unwinding roller 

1-9. (canceled)
 10. A method for producing a metallic riblet foil by a layer-type buildup by implementing the following steps at least once, the method comprising the steps of: providing a reference mold having a structure at least on one area corresponding to a female mold of at least one riblet structure; applying at least one layered portion made of a metal powder onto at least one area of the reference mold in such a manner that the at least one layered portion combines into a metal material; and detaching the at least one layered portion from the reference mold, which forms the metal riblet foil having a riblet structure at least on one area thereof.
 11. The method of claim 10, wherein the application of the at least one layered portion is achieved by spraying on a metal powder.
 12. The method of claim 10, wherein the at least one layered portion comprises a first layered portion of a metal powder comprising titanium or alloys of titanium, and a second layered portion of a metal powder comprising aluminum or alloys of aluminum.
 13. The method of claim 10, wherein a thickness of the at least one layered portion is variable.
 14. The method of claim 10, wherein the reference mold is reusable.
 15. The method of claim 10, wherein the reference mold is configured as a roller.
 16. The method of claim 15, wherein the at least one layered portion is adjustable by controlling a rotational speed of the roller.
 17. The method of claim 10, wherein a surface of the metallic riblet foil is at least partially refined. 